Observation with an electron microscope has been playing an important role in an analysis of micro constituents as a result of the recent development in the areas such as semiconductor devices and magnetic head elements, which have been experiencing continuous improvement in size toward minuteness. It is known that the spatial resolution of an electron microscope can be improved by decreasing the energy width of an incident electron beam with an energy filter, which is also called a monochrometer, so as to narrow an electron beam probe.
U.S. Pat. No. 5,097,126 discloses an energy filter which can adjust and stabilize the position of an electron beam on a slit, which has a sensor capable of detecting the electron beam, such as a fluorescent material which is able to detect a difference of intensity of an incident electron beam.
Also another U.S. Pat. No. 5,798,524 discloses an electron microscope, in which a predetermined percentage of an electron beam passing through the opening of a slit is defined and an automatic adjustment is performed for the electron microscope if the electron beam falls below the predetermined value. The automatic adjustment such as focusing of spectrum has the following steps: comparing the amounts of incident electron beam on upper and lower slit haves respectively; shifting the electron beam away from a slit half intercepting the greater amount and toward the other slit half intercepting the lesser amount; measuring the integrated intensity of an image of electron beam; and analyzing the surface plot of intensity.
Furthermore, the other U.S. Pat. No. 5,640,012 discloses an apparatus, in which the width of an opening of slit is controlled by shifting one of slit halves with an actuator according to an output signal detected by a light detector, which detects the light passing through the opening of slit.
As the width of the opening of slit is as small as some micron meters, it is difficult to let an electron beam pass through the center thereof.
Even after the electron beam passes through the opening successfully, a position of electron beam on the slit shifts due to the effect of instability of an electron beam source and the like, so that the electron beam is intercepted, which results in difficulty of observation with high resolution.
The apparatus according to the U.S. Pat. Nos. 5,097,126 and 5,798,524 is not able to compensate a deviation when the electron beam is dislocated so away from the opening of slit that the electron beam lies on a slit half. On the other hand, U.S. Pat. No. 5,640,012, which discloses a technique for controlling the width of a slit accurately, does not refer to another technique for controlling a position of electron beam on the slit so that the electron beam is positioned in the center of the opening of slit.